1. Field of the Invention
The present invention relates to an inverter, in particular, to an inverter which has a variable threshold potential.
2. Description of the Background Art
Conventionally, in a semiconductor integrated circuit, such as a flash memory, many inverters have been used in order to carry out a signal level detection, a signal transmission, a signal delay and the like. An inverter outputs an "H" level in case that an input signal level is lower than a threshold potential VTH, and outputs an "L" level in case that the input signal level is higher than the threshold potential VTH. However, for various reasons, there are some cases that the threshold potential VTH of a manufactured inverter diverges from its design value.
In the prior art, as shown in FIG. 22, a plurality (three in FIG. 22) of P-channel MOS transistors 121 to 123 and an N-channel MOS transistor 124 are formed so as to constitute an inverter. First, the P-channel MOS transistor 121 and the N-channel MOS transistor 124 configure an inverter, which functions to measure a threshold potential VTH of the inverter. On the basis of the measurement result, any one of a plurality of masks is selected and, if necessary, the P-channel MOS transistor 122 and/or 123 has been connected in parallel to the P-channel MOS transistor 121 in order that a threshold potential VTH of the inverter is adjusted.
However, in the prior art, a plurality of masks have to be prepared in advance, which results in taking significant amount of time and labor. Moreover, a wafer manufacturing process is temporally interrupted to measure a threshold potential VTH of a manufactured inverter, and a mask is selected and set based on the measurement result, which results in taking significant amount of time and labor.